This invention relates to a novel family of adsorbents based on an inorganic solid with a narrow and calibrated mesopore distribution which are agglomerated with a binder; these adsorbents are intended for the separation in the gas phase of compounds having different boiling points.
The adsorption technology is increasingly used industrially, for example for removing organic compounds present at a low content in a gas or liquid stream; it has the advantage of often being less expensive than other known industrial solutions, in particular when high volumes comprising impurities or trace amounts at a low concentration have to be treated and/or purified. Generally, a system operates with at least two columns filled with adsorbent, each column operating alternately in adsorption and desorption, the latter being intended to regenerate the adsorbent. The growth in this technology is driven to expand given the increase in regulations relating to the emission of VOCs (Volatile Organic Compounds), molecules which are encountered in all branches of the chemical industry and which can be chlorinated molecules or oxygenated molecules, such as ketones, or can belong to the aromatic family. The USEPA (United States Environmental Protection Agency) defines VOCs as stable compounds having a vapour pressure of greater than 0.1 mmHg (13.33 Pa) under standard temperature and pressure conditions.
The commonest technology for adsorbing VOCs in the gas phase consists in using a column filled with active charcoal over which the fluid to be treated is passed. When the concentration of the pollutant at the column outlet reaches a predetermined value, regeneration is carried out either with steam at a temperature close to 150-200xc2x0 C. or with a partial vacuum (technology of VRU (Volatile Recovery Unit) type). In the case of water-immiscible solvents, the method of regeneration of the active charcoal with steam is particularly advantageous as it makes it possible to recover the organic entity or entities, by separation by settling. However, it is known that the use of active charcoals has a number of disadvantages, such as the partial blocking of the pores by polymerization of reactive VOCs, a polymerization which is catalyzed by the surface ash of the charcoal; the active charcoal can also exhibit risks of ignition in the presence of oxygenated molecules when high heats of adsorption are involved. Finally, as the regeneration of the active charcoal is rarely complete, its capacity therefore decreases with the passage of the cycles, which forces the industrial operator to replace the active charcoal when its adsorption capacity has become too low. These disadvantages have prompted industrial users to find replacement solutions: the hydrophobic zeolites developed on this occasion, for example zeolites ZSM-5, have made it possible to avoid some of the disadvantages related to the use of active charcoal, in particular the risks related to inflammability (Blocki, S. W., Environ. Prog., 1993, 12, p. 226-230). However, their much higher price and their lower adsorption capacity than those of active charcoal are a brake on their industrial development and there thus exists a need for industrial adsorbents of moderate cost and with an adsorption capacity comparable to that of active charcoal while not exhibiting its handling hazards and having a superior regeneration capacity.
A family of mesoporous inorganic compounds with a narrow and calibrated distribution of the mesopores comprising in particular a mesoporous silica, the synthesis of which was disclosed for the first time in 1969 by Sylvania Electric Products Inc. in U.S. Pat. No. 3,556,725, the (alumino)silicon compounds of which family and more particularly the compound MCM 41 (for Mobil Composition Of Matter 41), a process for the synthesis of which is found in Nature, 1992, vol. 359, pp. 710-712, were much studied by Mobil at the beginning of the 1990s and have formed the subject of numerous patents and scientific articles, is now well known on the laboratory scale with regard to the porous structure, the synthetic conditions and the possible applications as catalyst and/or as adsorbent. In xe2x80x9cVOC Removal: Comparison of MCM 41 with Hydrophobic Zeolites and Activated Carbonxe2x80x9d by X. S. Zhao, Energy and Fuels, 1998, a comparative study is given regarding the respective adsorption and desorption properties of active charcoals, hydrophobic zeolites and MCM 41 as a powder for the removal of VOCs. From the plot alone of the isotherms of benzene, hexane and carbon tetrachloride, the authors conclude that the MCM 41 solids, because of their high mesoporous volume, which is markedly greater than that of hydrophobic zeolites, might constitute advantageous adsorbents for the removal of VOCs present at a high concentration in very wet gas flows but are only capable of adsorbing VOCs at a low concentration by a post-treatment intended to modify the diameter of the opening of their pores. Furthermore, the authors show that the desorption can optionally be carried out at a lower temperature than for the other adsorbents (of the order of 60xc2x0 C. instead of 100xc2x0 C.).
The adsorption of small organic molecules (methanol, butanol, toluene) on MCM 41 which has a variable level of aluminium and which is agglomerated by sintering was studied by T. Boger et al. (xe2x80x9cInfluence of the aluminium content on the adsorptive properties of MCM 41xe2x80x9d, Mesoporous Materials, 8, 1997, p. 79-91); the authors show that the adsorption of the organic molecules is subject to virtually no influence by the aluminium content of the MCM 41, which leads them to expect a capillary condensation mechanism, but also that the adsorption of the organic molecules is more unfavourably influenced in proportion as the sintering pressures increase. The adsorption of water is low and has a tendency to increase when the Si/Al ratio decreases; it remains in all cases low and partially irreversible. The adsorbent can be regarded as having a hydrophobic surface. The authors conclude therefrom that these adsorbents might be used in a humid atmosphere for removing VOCs present in gas streams at moderate to high concentrations but will have to be used in combination with a hydrophobic zeolite in order to obtain a high degree of purification, the hydrophobic zeolite adsorbing, for its part, the VOCs present at a low concentration. The partial irreversibility of the adsorption of water is confirmed by other authors (xe2x80x9cAdsorption Studies on Ordered Mesoporous Materialsxe2x80x9d by J. Janchen, Prog. In Zeolite and Microporous Materials, Studies in Surface Science and Catalysis, vol. 105, 1997, p. 1731).
Cycles of adsorption and of desorption of toluene and of 2-propanol were carried out on MCM 41 powder and compared with those obtained on a hydrophobic zeolite of ZSM 5 type (xe2x80x9cPressure Swing Adsorption of Organic Solvent Vapors on Mesoporous Silica Molecular Sievesxe2x80x9d by S. Namba et al., Prog. In Zeolite and Microporous Materials, Studies in Surface Science and Catalysis, vol. 105, 1997, p. 1891). The authors record a better working capacity of the mesoporous solid and a lower catalytic activity than that of the hydrophobic zeolite in the dehydration of propanol and conclude that a process of PSA type might make it possible to recover toluene or propanol vapours although they carried out their experiments at isopressure in desorbing under nitrogen.
All the conclusions of these abovementioned articles and patents are based on laboratory tests on powder products such as result from the hydrothermal synthesis and the use of these products in industrial plants cannot be envisaged, given the disadvantages relating to the handling and to the use of pulverulent materials.